Kayla Mueller, the U.S. aid worker who died earlier this year while being held hostage by Islamic State militants, was raped repeatedly by the group's leader Abu Bakr al-Baghdadi while in captivity in Syria, U.S. officials said on Friday.

The officials confirmed a report by ABC News, which said Mueller's family had been told by U.S. government officials that their daughter, who was 26 at the time of her death, had been sexually assaulted by al-Baghdadi.

The officials spoke to Reuters on the condition of anonymity. The White House declined to comment.

"We were told Kayla was tortured, that she was the property of al-Baghdadi. We were told that in June by the government," Kayla's parents, Carl and Marsha Mueller, were quoted as telling ABC News.

Islamic State said in February that Mueller, of Prescott, Arizona, was killed when Jordanian fighter jets bombed a building where she was being held outside Raqqa, a stronghold in Syria of the Islamist militant group. Jordanian and U.S. officials have expressed doubt about Islamic State's account of her death following 18 months as a hostage.

Mueller was taken hostage in August 2013 while leaving a hospital in Aleppo in northern Syria.

Al-Baghdadi personally brought Mueller to be imprisoned inside the home in Syria of Abu Sayyaf, a Tunisian Islamic State figure who was killed in a U.S. raid in May, counter-terrorism officials told ABC News over the past several months.

The information about al-Baghdadi's role in the captivity and sexual abuse of Mueller was drawn from many sources including U.S. interviews with at least two teenage Yezidi girls held as sex slaves in Sayyaf's compound and the interrogation of Sayyaf's wife, Umm Sayyaf, who was captured by U.S. forces in the raid in which her husband was killed, ABC News quoted the officials as saying.

Mueller went to Turkey in December 2012 to work for a Turkish organization providing humanitarian aid to Syrian refugees along the border with Syria.